Is it possible to run a script from an external editor without reloading the file?
For short or non-operator-based scripts, that's as efficient as you'll get. Currently there is no faster way than AltR-AltP to reload a plain script modified in external editor.

For longer operator-based scripts, you can use the following initial steps:


Convert the script to an add-on, by adding proper bl_info dictionary and turning your main code into operators (can be made faster by using a template).
Create a symbolic link of the script or its development path to Blender's alternate script path (set through User Preferences > File > Scripts). That way, it stays registered on Blender upgrade, and your development folder stays organized.

If your OS or filesystem doesn't support symbolic link, move the script to the alternate script path and develop from there.
Register the add-on.


After that, the script can be updated just by pressing F8 Reload Scripts in Blender. Longer preparation, faster in the long run.